The present invention relates to a magnetic flux guide and to a magnetoresistive transducer incorporating the flux guide and having two parts longitudinally separated by a head gap, as well as a magnetoresistive bar positioned facing the head gap. This transducer is more particularly intended for producing heads for reading and/or writing informations recorded in magnetic form on a random support such as a hard disk, tape, floppy disk, ticket, card, etc.
The transducer can be used for the detection and location of weak magnetic fields such as the geomagnetic field and the leaking of weak magnetic fields, e.g. associated with the presence of cracks in metallic systems.
Magnetoresistive components are generally in the form of a small magnetoresistive bar, whose ends are connected to current leads in order to bring about a current flow within the bar. In the presence of an external magnetic field, the magnetization M of the resistive bar is differently oriented so as to have the effect of modifying the resistivity of the material. A variation of the voltage then occurs at the terminals of the bar and the measurement of this voltage variation represents the value of the external magnetic field. It can also be representative of its direction.
Thus, by measuring the resistance of the bar it is possible to measure a magnetic field and conversely measure an electrical resistance by measuring a magnetic field.
The most modern magnetic transducers make use of magnetoresistive materials deposited in thin film form on an appropriate substrate. These films are etched in accordance with photolithographic processes in order to give them the shape of a bar.
The magnetic saturation field H.sub.s is defined as being the magnetic field applied to the magnetoresistive component and above which the intrinsic electrical resistivity .rho. of the component virtually no longer varies. The following relationship is defined: ##EQU1## in which H represents the value of the external magnetic field to be measured: .rho.(H=O) being the resistivity of the zero field magnetoresistive component and .rho.(H=Hs) its resistivity when the component is in the presence of a magnetic field equal to the saturation field. In addition, .DELTA..rho./.rho.=.DELTA.R/R, with R representing the resistance of the magnetoresistive component.
The sensitivity .varies. of the magnetoresistive structure and therefore that of a magnetoresistive effect transducer are defined by the variation of the magnetoresistance .DELTA.R/R obtained by applying a unitary external magnetic field and it satisfies the relation:
.varies.=(.differential.(.DELTA.R/R)/.differential.H(.sub.H&lt;Hs.
The shape of a magnetoresistive component or rather its dimensions influence the orientation of its magnetization. Thus, in the case of a thin magnetic film, the magnetization tends to be aligned in the plane of the film. Moreover, in the case of a long bar, the magnetization is generally oriented in accordance with said length. This amounts to a search for a minimum magnetic energy.
For a given magnetization direction, in the material appears a magnetic field known as the demagnetizing field H.sub.d or the form anisotropy field oriented in the direction opposite to that of the magnetization. The intensity of the demagnetizing field is a function of the geometry of the material in question and the direction taken by the magnetization within it.
FIG. 1 diagrammatically shows a magnetic bar having an elliptical section in order to illustrate the components of this demagnetizing field.
The values of the demagnetizing field coefficients are designated respectively N.sub.X, N.sub.Y and N.sub.Z in accordance with the directions X, Y and Z of an orthonormal coordinate OXYZ. When the magnetization M is parallel to the direction X, the demagnetizing field proves the relation H.sub.d =N.sub.X.M.
For a magnetoresistive bar of elliptical section assumed to be infinitely long and aligned in accordance with the direction X and having a thickness e in the direction Z and a height h in the direction Y, the following relations are proved: N.sub.x =0, N.sub.Y =e/(h+e), N.sub.Z =h/(h+e).
In the case of a bar having a rectangular section, whose thickness e is small compared with its height h, which is itself small compared with its length L, a first approximation consists of considering that: N.sub.X =0, N.sub.Y =e/h, N.sub.Z =1.
Thus, an external magnetic field H applied in the direction Y will be reduced within the magnetoresistive component by the demagnetizing field H.sub.d approximately equal in the latter case to (e/H)M, whereas said same external magnetic field applied in the direction X will not be reduced.
It should be noted that this approach no longer strictly applies as soon as the length of the component is comparable to its height or width. Thus, the demagnetizing field is liable to increase the value of the saturation field Hs and therefore decrease the sensitivity of the transducer.
Hitherto the magnetoresistive materials used for producing magnetic transducers were monolithic materials of the ferromagnetic type. They were mainly compounds based on iron and nickel (Fe.sub.19 Ni.sub.81, Fe.sub.20 Ni.sub.80) and compounds based on iron, nickel and cobalt (Fe.sub.15 Ni.sub.65 Co.sub.20, Fe.sub.5 Ni.sub.80 Co.sub.15, . . . ).
At present, new magnetoresistive materials are available. They consist of metallic multilayer magnetic structures (MMMS) constituted by a stack of magnetic layers separated by non-magnetic metallic layers having a thickness such that there is an antiferromagnetic coupling between the magnetic layers.
The investigated metallic multilayer magnetic structures vary. They are constituted by metals chosen from among cobalt, iron, nickel, alloys of iron and nickel, copper, chromium, silver, gold, molybdenum, ruthenium and manganese, as described in document (1) by H. Yamamoto and T. Shinjo (IEEE Translation Journal on Magnetics in Japan, vol. 7, no. 9, September 1992, "Magnetoresistance of multilayers", pp 674-684).
The MMMS's hitherto having the largest number of necessary properties (significant magnetoresistive effect, low saturation field, low coercivity, good annealing resistance) are constituted by FeNi layers separated by copper layers, as described in document (2) by S. S. P. Parkin ("Oscillations in giant magnetoresistance and antiferromagnetic coupling in [Ni.sub.81 Fe.sub.19 /Cu].sub.N multilayers", LLAppl. Phys. Lett. 60, No. 4, January 1992, pp 512-514) and document (3) by R. Nakatani et al (IEEE Transactions on Magnetics, vol. 28, No. 5, September 1992, "Giant magnetoresistance in Ni--Fe/Cu multi-layers formed by ion beam sputtering", p 2668-2670) or FeNi films separated by silver layers, as described in document (4) by B. Rodmacq et al, (Journal of Magnetism and Magnetic materials 118, 1993, pp L11-L16, "Magnetoresistive properties and thermal stability of Ni--Fe/Ag multilayers").
These new materials have the property of being highly magnetoresistive, i.e. having a ratio .DELTA..rho./.rho. from 10 to 20% and low saturation magnetic fields, i.e. below 40 kA/m.
In MMMS's the magnetoresistive effect corresponds to the rotation of magnetizations of each of the magnetic layers accompanied by a reduction in the electrical resistance during the application of an external magnetic field. In this case the saturation field Hs corresponds to the external magnetic field which it is necessary to apply in order to orient, in the same direction and sense, the magnetization of each of the different magnetic layers.
In the case of the ferromagnetic materials used in monolithic magnetoresistances, the resistance varies with the angle formed by the direction of the current density traversing the component and the magnetization direction in said material. The resistance is lowest when said angle is equal to .pi./2. Therefore it is not of interest in this case to apply the external magnetic field to be measured in any random direction, but perpendicular to the length of the magnetoresistive component. Conversely, parallel to the length of the bar, there is no magnetoresistive effect within monolithic ferromagnetic materials.
In modern MMMS structures, the variation of the resistivity and therefore the resistance is virtually independent of the angle formed between the direction of the field present within the magnetoresistive component and the direction of the current flowing through the magnetoresistive component.
However, this field is equal to the difference between the demagnetizing field H.sub.d and the external magnetic field to be measured H due to the fact that the shape and size of the MMMS's are such that, from the magnetic standpoint, they are generally anisotropic. The demagnetizing field coefficients are not identical in all directions of the component, its sensitivity varying with the direction of the magnetic field to be measured, as soon as the latter has an anisotropy of form.
The object of the invention is to produce a very sensitive transducer having a flux guide and making it possible to be free of the effects linked with the anisotropy of form (parallelepipedic), the magnetoresistive structure being used in the transverse mode.